fix(utils): discover nix submitEnv dynamically in dev checkout

Problem: In a dev checkout, `_nix_submit_cmd` is nil (only baked in by
the nix derivation). The uv fallback fails with code 127 on NixOS because
`uv` is not a bare system binary — it's only available via the FHS-wrapped
`cp-nvim-submit` script produced by `mkSubmitEnv`.

Solution: Add `discover_nix_submit_cmd` mirroring `discover_nix_python`:
runs `nix build #submitEnv --no-link --print-out-paths`, caches the result
in `stdpath('cache')/cp-nvim/nix-submit`, and sets `_nix_submit_cmd`.
`run_scraper` calls `setup_nix_submit_env()` before spawning submit.

lua/cp/scraper.lua

@@ -44,6 +44,10 @@ local function run_scraper(platform, subcommand, args, opts)
     return { success = false, error = msg }
   end
 
+  if subcommand == 'submit' then
+    utils.setup_nix_submit_env()
+  end
+
   local plugin_path = utils.get_plugin_path()
   local cmd
   if subcommand == 'submit' then